As a child, my brother would frequently challenge me to a game he called punch-for-punch. He’d let me hit him in the arm if he could hit me back just as hard. We’d exchange blows until one of us—the loser—quit. It wasn’t a long game; dull punches soon became bruising wallops. Being several years younger and many pounds lighter, I’d often concede quickly, fearing the next blow that, despite the game’s equally-hard rule, always felt more forceful than the last.